To count the days between two dates, subtract the earlier date from the later one. The result is the number of elapsed days, which does not count the start date. From January 15 to March 20, 2026 is 64 days. If you need to count both the first and last day, as for a hotel booking that runs “January 15 through March 20” or a medication taken each of those days, add one: 65 days. Getting that choice right matters more than the arithmetic.
The formulas
The date difference calculator does both and also breaks the span into years, months, weeks and days.
Counting by hand, month by month
- Days left in the start month: days in that month minus the start day.
- Full months in between: add the number of days in each.
- Days in the end month: the end day number.
- Add them up.
Example: January 15 to March 20, 2026
| Piece | Days |
|---|---|
| Rest of January (31 − 15) | 16 |
| February 2026 (not a leap year) | 28 |
| March 1 to March 20 | 20 |
| Elapsed days | 64 |
Inclusive count: 64 + 1 = 65.
Example across a year boundary: November 20, 2026 to February 10, 2027
| Piece | Days |
|---|---|
| Rest of November (30 − 20) | 10 |
| December | 31 |
| January | 31 |
| February 1 to February 10 | 10 |
| Elapsed days | 82 |
Since 82 = 11 × 7 + 5, that is 11 weeks and 5 days. November 20, 2026 is a Friday; adding 5 days to Friday gives Wednesday, which matches February 10, 2027.
Days in each month
| Month | Days | Month | Days |
|---|---|---|---|
| January | 31 | July | 31 |
| February | 28 (29 in leap years) | August | 31 |
| March | 31 | September | 30 |
| April | 30 | October | 31 |
| May | 31 | November | 30 |
| June | 30 | December | 31 |
The knuckle trick helps: make a fist and count months across your knuckles and the gaps between them. Knuckles are 31-day months; gaps are the shorter months. July and August are both knuckles, because you restart on the same knuckle. Check for leap years whenever your span includes February; how leap years work explains the rule.
Inclusive vs. exclusive: the fencepost trap
A fence 10 meters long with a post every meter needs 11 posts, not 10. Day counting has the same off-by-one issue. Which count to use depends on the question:
| Situation | Usual count | Example: start Mon, end Fri |
|---|---|---|
| Time elapsed, age, “how long ago” | Exclusive (elapsed) | 4 days |
| Hotel nights, rental periods returned on the end date | Exclusive | 4 nights |
| Events “from … through …”, daily doses, vacation days used | Inclusive | 5 days |
| Countdown “days until” an event | Exclusive | 4 days to go on Monday |
| Legal and court deadlines | Rules-based (see below) | Depends on the rule |
When a question is ambiguous, say which convention you used, for example “64 days (not counting the start date).”
Deadlines: counting forward from a date
Often you know the start date and a number of days and need the end date. Add the days, then check the weekday. Thirty days after October 11, 2026 is Tuesday, November 10, 2026. Ninety days after March 2, 2026 is May 31, 2026, a Sunday.
Formal rules often decide what happens next. Under the Federal Rules of Civil Procedure (Rule 6), when a period is stated in days you exclude the day of the event that triggers it, count every day including weekends and holidays, and include the last day; if the last day is a Saturday, Sunday or legal holiday, the period runs until the end of the next business day. So a 90-day deadline that lands on a Sunday would roll to Monday. Contracts, tax rules and state laws can define counting differently, so read the governing text. The date calculator adds or subtracts days, weeks, months or years from any date.
Counting months instead of days
Months are not a fixed length, so “how many months between two dates” needs a rule. The usual approach counts whole months from the start day number: January 15 to March 20 is 2 months and 5 days. Month-end dates are the tricky case. One month after January 31 has no matching day in February, so most calendars and spreadsheet functions such as EDATE land on the last day of February (February 28, 2026). Going the other way, from February 28 to March 31 is 1 month and 3 days by the day-number rule, even though both are month ends. When a contract says “one month,” check whether it means the same day number or the end of the month.
Calendar days vs. business days
“Days” usually means calendar days. “Business days” skips weekends and, depending on the context, holidays. Twenty business days is roughly four calendar weeks, not 20 calendar days. For those counts, see how to calculate business days or use the business days calculator.
Day counts in finance
Interest calculations use formal day-count conventions:
- Actual/365: the actual number of elapsed days over a 365-day year, common for consumer loans and savings. January 15 to March 20, 2026 is 64 days, or 64/365 of a year.
- 30/360: every month is treated as 30 days, common for corporate and municipal bonds and many mortgages. The same period is (3 − 1) × 30 + (20 − 15) = 65 days, or 65/360 of a year.
The difference is small for one period but matters for accrued interest on large balances.
Spreadsheet formulas
With the start date in A1 and the end date in B1:
| Count | Formula |
|---|---|
| Elapsed days | =B1-A1 or =DAYS(B1,A1) |
| Inclusive days | =B1-A1+1 |
| Weeks and days | =INT((B1-A1)/7) and =MOD(B1-A1,7) |
| Business days (inclusive) | =NETWORKDAYS(A1,B1) |
| Days from today | =B1-TODAY() |
Format the result as a number, not a date, or a spreadsheet may display something like “3/4/1900.”
Quick checks for your answer
- A full non-leap year from a date to the same date next year is 365 days; any span containing February 29 adds one.
- Seven days later is always the same weekday; use this to sanity-check long counts.
- From October 11 to December 25, 2026 is 75 days; the days until calculator and countdown timer track spans like this automatically.
To count your own age this way, see how to calculate your age in days.
Frequently asked questions
How many days are between January 15 and March 20, 2026?
64 days, counting elapsed days (the start date is not counted). That is 16 days left in January, 28 in February and 20 in March. If you count both January 15 and March 20, it is 65 days.
What does inclusive mean when counting days?
Inclusive counting includes both the start date and the end date. It always gives one more than the elapsed-day count. Monday through Friday is 4 days elapsed but 5 days inclusive.
How do I count days between dates in Excel?
Subtract the cells: =B1-A1 returns elapsed days when A1 is the start date and B1 the end date. =DAYS(B1,A1) does the same. Add 1 for an inclusive count, and use NETWORKDAYS for business days.
How many days are in 2026?
365. 2026 is not a leap year; the next leap year is 2028, which has 366 days because it includes February 29.
Is 30 days from today the same as one month from today?
Not usually. Months have 28 to 31 days, so one month from October 11 is November 11 (31 days), while 30 days from October 11 is November 10. Contracts and laws specify which one they mean.